1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to a turbine airfoil tip with cooling.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine uses a compressor that produces a compressed air fed into a combustor and burned with a fuel to produce a hit gas flow. This hot gas flow is passed through a turbine which progressively reduces the temperature of the hot gas flow and converts the energy into mechanical work by driving the turbine shaft. Designers are continuously looking for ways to improve the engine performance. Raising the temperature of the hot gas will increase the efficiency of the engine. However, the temperature is limited to the material properties of the first stage vane and blade assembly. Designers have come up with complex cooling passages for cooling these critical parts in order to allow for the hot gas flow temperature to exceed the melting temperatures of these parts.
Another way to improve the performance of the engine is to reduce the leakage flow between the rotor blade tip and the outer shroud that forms a seal with the tip. Because the engine cycles through temperatures, the tip clearance varies. Sometimes, the tip touches against the shroud, causing rubbing to occur. Rubbing can damage the blade tips. Providing a larger tip clearance will reduce the chance of rubbing, but will also allow for more hot gas flow to leak across the gap and expose the blade cap to extreme high temperature. Cooling of the blade tip is required to limit thermal damage. Separate blade tip cooling passages have been proposed.
Designers have proposed using a squealer tip rail to reduce the blade tip leakage and also provide for rubbing capability for the blade. A squealer tip provides for a thin rail extending from the blade top to form the seal between the shroud. The tip rail is thin and therefore does not provide much surface area against the shroud when rubbing occurs. Thus, with a squealer tip, the effect of rubbing is minimized. FIG. 1 shows a prior art blade with a squealer tip cooling arrangement. The blade has a pressure side 12, a suction side 13, and a top 14 with a tip rail 15 extending along the top edge from the trailing edge around the leading edge before stopping short of the trailing edge on the pressure side 12. Film cooling holes 17 on the pressure side 12 and tip cooling holes 16 on the top provide cooling air for the blade. The squealer tip is formed by the tip rail 15. Secondary leakage flow 21 over the tip is shown and turns into a vortex flow 22 on the blade suction side 13.
FIG. 2 shows a prior art blade with a cooling arrangement for the suction side 13 with a tip rail 15. Suction side tip peripheral film cooling holes 18 are arranged along the suction side near the tip 15. A very hot gas vortex flow 23 is created by the tip configuration on the suction side toward the trailing edge. The suction side blade tip rail 15 is subject to heating from three exposed sides, and therefore cooling of the suction side squealer tip rail 15 by means of discharge row film cooling holes along the blade pressure side peripheral and at the bottom of the squealer floor becomes insufficient. The is primarily due to the combination of tip rail geometry and the interaction of hot gas secondary flow mixing. The effectiveness induced by the pressure side film cooling and tip section convective cooling holes is very limited.
It is therefore an object of the present invention to provide improved blade tip cooling in order improve engine efficiency and increase part life of the blade and shroud.